I have in fact read both, but not in many years for the Miller. I think A Fire upon the Deep is quite good (nearly everything by Vinge is good), but I find A Deepness in the Sky (in the same series, written later but taking place earlier and standing on its own) even better, for its wonderful aliens, its interesting treatment of the problem of translation, its conflicting human cultures, and its tense narrative (essentially the final third of the book is a sustained climax).
My favorite single body of science fiction is the twelve Heinlein juveniles: Rocket Ship Galileo, Space Cadet, Red Planet, Farmer in the Sky, Between Planets, The Rolling Stones, Starman Jones, The Star Beast, Tunnel in the Sky, Time for the Stars, Citizen of the Galaxy, and Have Space Suit—Will Travel. These were written as boys’ books, but the resulting constraints kept Heinlein from riding some of his hobby horses, and to my mind his portrayal of women is much better than was usual in the 1950s—I’m thinking in particular of a passage in The Rolling Stones where Edith Stone, an MD retired to raise a family, learns that a ship in a nearby orbit has an unknown disease that has killed its medical officer, and starts getting ready to go and give help; when her husband objects she simply says, “Roger, I am a doctor” and he backs down.
Other possibilities:
Lois McMaster Bujold, Barrayar, a sequel to Shards of Honor, and often included with it in Cordelia’s Honor: probably the closest that science fiction has come to grand opera.
Octavia Butler, Mind of My Mind, the first of a series, which I think is really brilliant in its treatment of psi powers; it’s in the spirit of Thucydides’ line “concerning gods we have the belief, and concerning humans the certainty, that each does what is in his power: the strong do what they can, the weak do what they must.”
C.J. Cherryh, The Pride of Chanur, a classic SF adventure story with really good aliens, starting when a ship of the leonine hani picks up a refugee belonging to an unknown species.
Michael Flynn, The Wreck of The River of Stars, the story of a disaster in outer space, which is equal parts a study in industrial failure and a classical tragedy.
Robert Heinlein, Double Star: His first novel to win the Hugo Award, and a classic example of his “the man who learned better” plot structure.
Donald Kingsbury, Courtship Rite, which I think is the best science fiction novel I’ve read; it has a profoundly alien culture, and did the trick of getting me into the minds of its characters so well that when Oelita the Heretic makes amends to her world’s God it made total sense to me; and it also has a consistent intellectual theme of optimization in domains ranging from biology to public choice to ethics.
Tim Powers, Declare, is a tightly plotted World War II spy thriller (impressively so for Powers, whose plots aren’t usually anything like that tight) that turns on what could be taken as supernatural horror but has a science fictional rationale.
Jack Williamson, Darker than You Think, another case of apparent supernatural horror with a science fictional rationale; its underlying logic seems to be akin to psychoanalysis, but with a Nietzschean rather than Freudian unconscious tied to a premise about human prehistory.
There are others that I like a lot, but I think those will do for a start. I hope you find some of them worthwhile.